Digital electronic systems are generally fabricated from a number of integrated circuit chips that mount on printed circuit boards. The printed circuit boards carry traces that facilitate the transfer of signals representing digital information among the chips and other components on the boards. A digital system may comprise a number of boards mounted in one or more cabinets, and it is generally necessary to interconnect the boards to facilitate transfer of signals among the boards. Within a single cabinet, the wires in the form of backplanes or cables may be used to transfer signals among the broads, and between cabinets cables are typically used as transmission lines to transfer the signals.
In particular between cabinets, but also between boards and in some cases between chips on the same board, "differential" signaling is typically used. In differential signaling, two physical signals are transmitted together as a "differential pair" to facilitate the transfer of a single bit of information, with the logical state of the information bit being represented by the difference in voltage levels between the signals. If, for example, one signal of the differential pair is at a high voltage level and the other is at a low voltage level, the logical state of the information bit may be taken as asserted, or a "one." On the other hand, if the states of the signals are reversed, the logical state of the information bit may be taken as negated, or a "zero."
Differential signaling may be used for a number of reasons. Differential signaling may be used to reduce the likelihood that information transferred can be corrupted due to some types of noise, or undesirable voltage changes, which may be induced in the lines carrying the signals between the signal driver and the receiver. The lines which carry differential signal pairs are typically routed adjacent each other, and if noise is induced in one line it will likely also contemporaneously be induced in the other and with the same amplitude, resulting in what is generally known as "common-mode noise." Since the state of the information bit is represented by the difference between the voltage levels in the two lines, the noise voltage in the two lines will cancel.
In addition, differential signaling may be used when routing lines between components of a digital system that are powered by different power supplies, particularly if such components are in separate cabinets. In such systems, the voltages provided by the power supplies may be different, in which case the absolute voltage levels of the signals between the components may also be different. This is a particular difficulty in connection with components of a system that are housed in diverse cabinets. Since in differential signaling the state of the information bit is represented by the difference between voltage levels in the two lines carrying the differential signal pair, rather than by their absolute voltage levels, such offsets, which are termed "common-mode voltage shift," will be present in both lines of the differential signal pair and canceled when the voltage difference is determined.
The rate at which information bits can be transmitted in a digital system depends on a number of factors, with a primary factor being the rate at which transitions in a signal's voltage level can be made to occur on the line carrying the signal. However, a sharp transition can also cause a problem, in particular resulting in generation of noise, known as "ringing," at the beginning of a transition. That form of noise is one type of "differential noise," that is, noise that is individually generated and coupled over each line of the differential signal pair. In differential noise, the amplitude and timing of the noise are not necessarily the same on both lines and thus can not be corrected by taking the difference between voltage levels in the lines carrying a differential pair.
Another problem arises in some very large digital systems, particularly those with a large number of interconnection lines, namely, the maintenance of the lines among components to ensure that correct information is being transferred. In some large digital systems, the components may be connected by hundreds or even thousands of lines, and, if a line is faulty, incorrect information may be transferred. The problem may be exacerbated in a system that employs differential signaling, particularly if only one line of a differential signal pair is faulty. When that occurs, the information bit represented by the differential signal pair may generally be correctly received, with erroneous bits being received only intermittently. Thus, in such cases it may be difficult to determine even the fact that one line of the differential signal pair is faulty.